Recently, a light emitting element having a high luminous efficiency and a photoelectric conversion element having a high signal-noise ratio (SNR) have been developed to meet various needs. These devices have been combined to develop an advanced sensor module. For one example, there has been a gas sensor that employs a non-dispersive infrared (NDIR) method. The conventional gas sensor of the NDIR method has been used including a tungsten lamp as an infrared light source and a thermopile as a light receiving portion.
However, for achieving downsizing and low power consumption, a configuration where a Light Emitting Diode (LED) is employed as a light source and a quantum type infrared device is employed as a light receiving portion has been becoming a future standard structure of the gas sensor of the NDIR method. Higher sensitivity and higher SNR of the light receiving portion and higher luminous efficiency of a light emitting portion is achieved by using, for example, a narrow gap compound semiconductor material in Groups III-V as the light receiving portion and the light emitting portion.
An optical device such as the quantum type infrared device and the LED has been specifically attracting attention from a point of achieving tremendous low power consumption of a gas sensor module. These optical devices have another great feature that the optical devices can be sealed by a resin mold. Sealing by the resin mold ensures easily downsizing the light emitting portion and the light receiving portion, and ensures improving respective performances of the light emitting portion and the light receiving portion, that is, enhancing the luminous efficiency of the LED and enhancing the SNR of the light receiving portion achieve enhancing a resolution and the SNR of the gas sensor that employs these optical devices.
In addition to improve the performances of the light emitting portion and the light receiving portion, future gas sensors using NDIR method will require a long-term stability. In aspect of the long-term stability, a signal drift caused by stress and heat is apprehended.
That is, in a package (hereinafter also referred to as a resin-molded package) where an optical device is molded by a sealing resin, a stress variation caused by moisture absorption and heat of the resin used for the mold possibly influences on a variation of an amount of luminescence and a variation of light receiving sensitivity of the optical device such as the infrared device and the LED. Depending on the specification, especially, a gas sensor that requests resolution for a ppb order causes a significant error on a measurement result of gas concentration even when an optical signal level slightly varies due to an influence of a disturbance.
As a method for reducing the stress variation, there has been proposed a method such that, for example, transmission means is disposed on a region for receiving or emitting light on a semiconductor chip and the other region is sealed with an insulating resin that includes a filler, so as to match a thermal expansion coefficient of a material of the semiconductor chip with a thermal expansion coefficient of the resin, thus suppressing the stress variation (for example, see PTL 1).
For example, PTL 2 discloses a method such that a protective film is disposed to cover a substrate and an infrared light receiving element mounted on the substrate, that is, a protective film for protecting the infrared light receiving element is disposed between the sealing resin of the package and the infrared light receiving element, thus protecting the infrared light receiving element from the stress of the sealing resin. PTL 2 also discloses a method such that the protective film is disposed to protect the infrared light receiving element for reducing the influence of the stress, and furthermore, a cavity region is disposed between the protective film and the sealing resin to improve photoelectric conversion efficiency, thus ensuring the reduction of the influence of the stress.